Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as, WiFi Direct. These systems operate in unlicensed spectrum.
Recently, Device-to-Device (D2D) communication, as an underlay to cellular networks, has been proposed as a means to take advantage of the close proximity of communicating devices, i.e. user equipments, UEs, and at the same time to allow these UEs to operate in a controlled interference environment. In this case, close proximity may typically refer to less than a few tens of meters, but sometimes even up to a few hundred meters.
This direct mode or device-to-device, D2D, communication may demonstrate a number of potential gains over traditional cellular communication. One of these potential gains is capacity. Radio resources, such as, e.g. Orthogonal Frequency-Division Multiplexing, OFDM, resource blocks, between the D2D and cellular layers may be reused, resulting in reuse gains. Also, the D2D link uses a single hop between the transmitter and receiver points as opposed to the double-hop link via a cellular access point, resulting in hop gains. Another potential gain is peak rate. Because of the proximity, and potentially favourable propagation conditions for the D2D link, high peak rates are possible to achieve, resulting in proximity gains. A further potential gain is latency. When UEs communicate over a direct D2D link, forwarding via the cellular access point is short-cut and the end-to-end latency between the UEs is decreased.
It has further been suggested that such D2D communication should share the same spectrum as the cellular system, for example, by reserving some of the cellular uplink resources for D2D communication purposes. That is, locating D2D communication on cellular uplink, UL, resources in a way such that Time-Division Duplex, TDD, is the duplex transmission scheme of the D2D communication. This means that the cellular UL resources would be allocated for D2D communication transmission in both upstream and downstream directions between each D2D pair of UEs in a Time-Division Multiplexed, TDM, manner.
This D2D communication may be established and performed by the D2D pair of UEs only, which is commonly referred to as an ad-hoc mode D2D communication, or be assisted by the network, also referred to as a network-assisted D2D communication.
In the latter, a cellular network may, for example, assist a D2D connection by establishing security of the D2D link and/or partly or fully controlling the setup of the D2D connection, such as, e.g. UE/peer discovery and resource allocation. Further, a cellular network may also assist the D2D communication by controlling the interference environment. For example, if using a licensed operator's spectrum for the D2D communication, higher reliability may be provided than when operating in an unlicensed spectrum. To further assist the D2D connection, the cellular network may also provide synchronization and/or partial or full Radio Resource Management, RRM. The RRM may here, for example, comprise time and/or frequency resource allocation for the D2D communication. However, this network-assisted D2D communication requires that the radio network node is configured with a Radio Access Technology, RAT, according to the LTE standard, that is, a RAT that can be used for D2D communication.
However, today, if there is no coverage provided by a LTE cellular network, the D2D communication will fall back to ad hoc mode D2D communication.
It should also be noted that most of the current LTE UEs support multi-mode communication, i.e. the LTE UEs are capable of communicating also with non-LTE cellular networks, that is, cellular networks utilizing other RATs which cannot be used for D2D communication, such as, e.g. GSM, WCDMA, HSPA.
WO2013055271 discusses D2D communication between UEs when the UEs are in the coverage of a network that lack D2D capabilities. Here, a UE may switch to a neighbouring cell/network with D2D capabilities, and then set up a D2D communication with another UE.
US20130150051 disclose the use of a D2D registration server for UE to register to. The D2D registration server function may in turn connect to different RAN systems to control a D2D connection.
EP2591619 disclose different triggers for a D2D UE/peer discovery procedure utilizing either a push mode, i.e. discovery is triggered periodically, or pull mode, i.e. discovery is triggered by a UE/peer request signal from a UE/peer. The D2D discovery procedure can be implemented with or without assistance from the network.